***************************
*      2.4.0 NEWS         *
***************************
1. edit the proportion_index function into a new function low_frequency_op
2. change the DESCRIPTION file
3. modify the man and vignette file accordingly
4. change the name of the optimal_codon_statistics to op_highly_stats

***************************
*      2.0.0 NEWS  	  *
***************************
1. add three new functions akashi_test, op_corre_CodonW and op_corre_NCprime.
2. change the name of three function proportion_index, op_highly and optimal_codon_statistics
3. split one function optimal_codons into two functions: op_highly and optimal_codon_statistics
4. change corresponding files, including DESCRIPTION, NAMESPACE, NEWS, manuel files and vignettes

***************************
*      0.99.4 NEWS  	  *
***************************
1. update R version

***************************
*      0.99.3 NEWS  	  *
***************************
1. change VignetteEngine error

***************************
*      0.99.2 NEWS  	  *
***************************
1. add a function genomic_gc3 in the package
2. change vignette


***************************
*      0.99.1 NEWS  	  *
***************************

1. import unlist function from BiocGenerics package
2. change the name of optimal_mutation_index to optimal_index, remove the conditional
test for genomic GC content should be lower than 50%
